The present invention relates generally to methods for producing vinyl chloride resins, and, more especially to a method for producing an improved vinyl chloride resin, employing a particular partially saponified polyvinyl alcohol stabilizer. The particular polyvinyl alcohol exhibits excellent dispersion-stabilizing activity in the suspension polymerization, in aqueous medium, of vinyl chloride monomer or a mixture of vinyl chloride as a major monomeric component and one or more monomers copolymerizable therewith as a minor monomeric component. In another aspect, the invention is directed to a vinyl chloride resin which has an improved particle size distribution with a minimum of coarse beads, excellent processability and a significantly reduced residual vinyl chloride content, the resin being obtainable by the aforementioned method.
One commonly employed commercial method for producing vinyl chloride homopolymer or copolymer resins, both of which are widely utilized, is that process termed suspension polymerization, whereby vinyl chloride monomer alone or a mixture thereof with one or more copolymerizable monomers is dispersed by addition to a stirred aqueous medium, in the presence of a suspension stabilizer, the polymerization reaction being conducted with the aid of an oil-soluble catalyst. Among the factors generally governing the quality of the product resin are the degree of polymerization, monomer-to-water ratio, temperature, the amount of catalyst, the engineering of the autoclave, the rate of agitation, and the type and amount of suspension stabilizer employed. However, it has been found from experience that the type of suspension stabilizer is a most critical factor. Known dispersing agents for the polymerization of vinyl chloride monomer include water-soluble polymers such as cellulose derivatives, e.g., methyl cellulose, carboxymethyl cellulose, etc., and partially saponified polyvinyl alcohols, the latter being the more often employed in current commercial processes.
The desired characteristics of the suspension stabilizer, i.e., dispersing agent, employed in this polymerization reaction include a high dispersing power even when used in a small amount and, an ability to yield a product polyvinyl chloride resin having a particle size distribution as uniform as possible with a small mean particle size, adequate porosity and improved processability, less susceptible to the formation of fish eyes on further processing, and a significantly reduced residual vinyl chloride monomer content. However, the resins in the vinyl chloride series produced according to the aforementioned prior art methods employing known suspension stabilizers are not satisfactory in porosity, particle size distribution and mean particle diameter, and yield a high vinyl chloride residual monomer content further contributing to poor overall processability.
Regarding the physical properties of polyvinyl alcohols, the average degree of polymerization, the average degree of saponification, etc., must be within certain ranges appropriate for the stabilization of dispersions. It is known that modified polyvinyl alcohols containing carbonyl groups within the respective molecules, obtainable by hydrolyzing the polyvinyl esters resulting from polymerization in the presence of an aldehyde or ketone, are suitable as suspension stabilizers for the suspension polymerization of vinyl chloride monomer.
Heretofore attempts have been made to meet the above performance requirements by selecting a suitable combination of such factors as the degree of polymerization of polyvinyl alcohol, the degree of saponification thereof and the distribution of residual acetyl groups. But, the attempts have met with only limited success. Thus, it is generally known that the use of a partially saponified polyvinyl alcohol having a high degree of polymerization and a high degree of saponification provides a high dispersing power and, therefore, a product resin having a uniform distribution of relatively small particle sizes, but the resin is inadequate in porosity and in the ability to absorb plasticizers. Conversely, a low degree of saponification makes for excellent porosity and an improved rate of plasticizer absorption but the low dispersing power of such polyvinyl alcohol leads merely to the production of vinyl chloride resin particles which are large in diameter and exhibiting a high residual vinyl chloride monomer content.